1. Field of the Invention
The present invention generally relates to a laser driving unit and an image forming apparatus that includes such a laser driving unit.
2. Description of the Related Art
Conventional semiconductor laser driving circuits may be roughly categorized into a zero-biased (or non-biased) type and a biased type. The zero-biased type semiconductor laser driving circuit sets a bias current of a semiconductor laser to zero, and drives the semiconductor laser by a pulse current corresponding to an input signal. Examples of the zero-biased type semiconductor laser driving circuit are proposed in Japanese Laid-Open Patent Publications No. 4-283978 and No. 9-83050, for example.
When driving a semiconductor laser having a relatively large threshold current by the zero-based type semiconductor laser driving circuit, it takes a certain amount of time until a carrier concentration sufficient to cause laser oscillation is generated, even when a driving current corresponding to the input signal is applied to the semiconductor laser. As a result, delay is generated in the light emission from the semiconductor laser. The light emission delay does not cause a serious problem if a pulse width of the input signal is sufficiently wide compared to the light emission delay time such that the light emission delay is negligible. However, when the semiconductor laser is to be driven at a high speed in order to realize high-speed operation in equipments such as laser printers, optical disk drives, and digital copying apparatuses, for example, a pulse width of the light that is emitted from the semiconductor laser may not be made wide to an extent desired.
On the other hand, the biased type semiconductor laser driving circuit sets a bias current of the semiconductor laser to a threshold value. The semiconductor laser is driven by constantly flowing the bias current and adding a pulse current corresponding to the input signal. An amount of current corresponding to an oscillation threshold value is supplied to the semiconductor laser in advance when using the biased type semiconductor laser driving circuit, and thus, the light emission delay may be substantially eliminated. However, the semiconductor laser constantly emits light in a vicinity of the oscillation threshold value (for example, at 200 μW to 300 μW) even when the semiconductor laser is not driven. Hence, when the semiconductor laser driven by the biased type is used in optical communication, for example, the extinction ratio becomes small. In addition, when the semiconductor laser driven by the biased type is used in an image forming apparatus such as the laser printer and the digital copying apparatus, for example, a banding type noise may appear in a background portion of the paper that is subjected to the printing or copying.
Hence, in the optical communication, the zero-biased type semiconductor laser driving circuit is used in order to reduce the deterioration of the extinction ratio.
On the other hand, there are demands further improve the resolution of the laser printer, the optical disk drive, the digital copying, and the like. Accordingly, there are proposals to use a red semiconductor laser that emits light having a wavelength of 650 nm or, an ultraviolet semiconductor laser that emits light having a wavelength of 400 nm, for example. These semiconductor lasers require more time until the carrier concentration sufficient to cause laser oscillation is generated, when compared to the conventionally used semiconductor lasers that emit light having wavelengths on the order of 1.3 μm, 1.5 μm, and 780 nm. For this reason, even if the semiconductor laser that emits light having the wavelength on the order of 650 nm or 400 nm is driven the biased type, the pulse width of the light that is emitted from the semiconductor laser may not be made wide to the extent desired.
Further, when a low tone is to be reproduced on the paper by the image forming apparatus using the light having a narrow pulse width on the order of several ns (nano-seconds) or less, for example, the output of the semiconductor laser may not reach its peak intensity. Consequently, the low tone that is actually reproduced may become lower than originally intended, and a correct tone reproduction may be difficult to achieve.
In order to suppress the problem related to the tone reproduction, a Japanese Laid-Open Patent. Publication No. 5-328071 proposes correcting the tone of a low tone region by superimposing a differential pulse on a rising edge of the driving current. However; this proposed technique cannot control a peak of the differential pulse, and the semiconductor laser may break down. In addition, because the time in which the differential pulse is superimposed on the driving current is dependent on a differential waveform, the tone of the low tone region may only be improved at an initial stage of the correction, and the gradation representation may not increase linearly after the initial stage of the correction.
In addition, a Japanese Patent No. 3466599 proposes a correction using a bias current, an oscillation threshold current, a light emission current, and an auxiliary driving current, in order to suppress the problem associated with the proposed technique that superimposes the differential pulse on the driving current. According to the proposed technique that uses four currents for the correction, the driving current may have a waveform approximating an ideal rectangular waveform. However, depending on the settings of the bias current and the oscillation threshold current, the pulse width of the optical waveform may become narrower than the pulse width of the input signal.
In a case where the semiconductor laser includes a plurality of light sources, a parasitic capacitance of a wiring differs among the light sources, because a wiring length between a driving circuit and each light source and a wiring length within each light source differ among the light sources. Thus, the narrowing of the pulse width of the optical waveform may differ among the light sources due to the parasitic capacitance that differs among the light sources. The difference in the quantities of light (or luminous energies) emitted from the light sources tends to increase as the narrowing of the pulse width of the optical waveform increases due to the effects of the parasitic capacitance that differs among the light sources.
In the image forming apparatus, the semiconductor laser that is popularly used may be a laser diode, a semiconductor laser array, a VCSEL (Vertical Cavity Surface Emitting Laser), and the like. An optical waveform response characteristic of the semiconductor laser may differ depending on the structure, wavelength characteristic, output characteristic, and the like of the semiconductor laser.
When the semiconductor laser is mounted on a circuit board together with the driving circuit, the wiring is formed between the semiconductor laser (or each of the light sources included in the semiconductor laser) and the driving circuit, and within a package of the semiconductor laser. The wirings include varying factors that affect the optical waveform response characteristic, such as the parasitic capacitance, inductance, and resistance components. Particularly in the case of a semiconductor laser having a relatively large package size, the parasitic capacitance may increase considerably, and the resistance component may increase considerably depending on the wavelength region. In other words, the optical waveform response characteristic of the semiconductor laser may vary depending on such varying factors.
For example, the differential resistance of the red semiconductor laser in the 650 nm wavelength region is large compared to the infrared semiconductor laser in the 780 nm wavelength region. Hence, a high-speed response of the optical waveform may not be obtained from the semiconductor laser and the response of the optical waveform may be slow, depending on the structure of the driving circuit, the circuit board, and the like.
In addition, in the case of the VCSEL, the differential resistance is extremely large compared to the edge-emitting semiconductor laser having the differential resistance on the order of approximately several hundred Ohms, because the structure of the VCSEL differs considerably from the structure of other infrared edge-emitting semiconductor lasers. For this reason, because of the time constant generated by the terminal capacitance of the VCSEL itself, the parasitic capacitance of the circuit board (or substrate) on which the VCSEL is mounted, the terminal capacitance of the driving circuit mounted on the circuit board, and the differential resistance of the VCSEL, the optical waveform with a high-speed response may not be obtained even if the VCSEL itself has a device characteristic and a cutoff characteristic that enable a high-speed modulation, after the VCSEL is mounted on the circuit board.
When the semiconductor laser having the above described varying factors include a plurality of light sources, the response characteristic of the light source may differ considerably among the light sources. The different response characteristics of the light sources cause differences in the oscillation delay and a transition time in which the light emission quantity varies. As a result, when the semiconductor laser is used in the image forming apparatus, for example, these differences may cause inconsistencies in the tone reproduction, color registration error, and the like.
Moreover, in the semiconductor laser, the amount of change in the light emission level with respect to the amount of change in the driving current differs between a LED (Light Emitting Diode) region in which the driving current changes from zero to the threshold value, and a LD (Laser Diode) region in which the driving current is greater than the threshold value. For this reason, when driving the semiconductor laser in the image forming apparatus by increasing the driving current from a state in which the applied bias current is less than the threshold value to the light emission stage, the oscillation delay may occur with respect to the driving current because of the low light emission level in the LED region.